1. Field of the Invention
This invention relates to a blood pump apparatus for use in an extracorporeal blood circulation system.
2. Prior Art
Extracorporeal circulation systems are used for circulating the blood of a patient through a treatment device such as an artificial lung or an oxygenator. In such a system, the treated blood is returned to the body of the patient by a blood pump apparatus. And it is a common practice in the art to use as such a blood pump apparatus a blood pump of the type that feeds the blood in a flexible tube by squeezing the tube by one or more rollers, although other types of blood pumps such as a reciprocal type and a diaphragm-type are also known in the art.
Generally, when it is required to use an auxiliary blood pump, an amount of flow of the blood required to be supplied into the body of the patient per a unit period of time is first determined in accordance with the condition of the patient, and then the speed of the pump is set to such a value that the required amount of flow of the blood (ml/min) is obtained. In this case, it is considered desirable that the blood is pumped not in a continuous manner but in a pulsatile manner as is done by the heart.
For this reason, when a conventional roller-type blood pump is used to produce a pulsating flow of the blood, the rotation speed of the pump is previously set to such a value that a desired flow rate of the blood is obtained, and then the pump is controlled to operate intermittently by means of a switch circuit to produce a pulsating flow of the blood. In this case, the switch circuit is so arranged that it closes to begin to operate the pump a predetermined period of time (hereinafter referred to as delay time) after each of R waves of electrocardiogram of the patient is detected, and that it continues to close for a predetermined percentage of time length of the interval between two adjacent R waves (the interval is hereinafter referred to as R-R interval). The reason why the closures of the switch are so determined is as follows:
As shown in FIG. 1, an electrocardiographic wave Wh includes peaks called P, Q, R, S and T waves, and wherein the P wave indicates the excitation of the atria; Q, R and S waves the process of excitation of the ventricles; and T wave the calming down process of the excitation of the ventricles. Peak of the pressure Wv of the interior of the left ventricle appears during the process indicated by the Q, R and S waves when the heart contracts to pump the blood into the aorta. More specifically, the peak of the pressure Wv appears before the T wave but slightly after the S wave. The aortic valve closes during the period A shown in the same figure, which comes after the T wave, when the pressure of the interior of the left ventricle abruptly decreases. And if the blood is subjected to a pressure by the pump before the closure of the aortic valve, the pressure is transferred via the aortic valve to the left ventricle, so that the heart is subjected to an unnecessary and dangerous load.
Accordingly, in the case where such a roller-type blood pump as described above is used for pumping blood into the body of a patient, the aortic valve must have been closed before the pressure of the pumped blood reaches the aorta. This is the reason why the aforementioned delay time should be provided. And if the pump is operated to rotate in such a manner that the blood pressure is increased after the aortic valve has been closed, a sufficient blood pressure can be applied to the proximal portion of the aorta (the portion of the aorta near the aortic valve). This causes the blood to be forcibly introduced into the coronary arteries, which opens to the proximal portion of the aorta at its proximal end to supply the blood to the cardiac muscle, so that a diastolic augmentation can be attained. Incidentally, the R--R interval is a specific value to each patient and is constant, and therefore the duration of each continuous rotation of the pump required to feed an appropriate amount of blood can be determined as a certain percentage of time length of the R--R interval.
The conventional roller-type blood pump described above is so constructed that the rotation speed and the interval, delay time and duration of each operation of the pump are adjusted to desired values manually and independently from each other. And therefore, when using the conventional roller-type pump, the operator has to first adjust the rotation speed of the pump to a selected value at which a desired flow rate of the blood will be obtained and then has to adjust in accordance with the R-waves the timings of the switch circuit so that the desired interval, delay time and duration of each operation of the pump are obtained. Thereafter, the operator further has to readjust the rotation speed of the pump so that the desired flow rate of the blood is obtained, since the flow rate of the blood has been varied by the adjustment of duration of operation of the pump. And in the case where the desired flow rate of the blood and the desired operation timings of the pump have not accurately been obtained by the above adjustment procedure or the condition of the patient has been changed, the operator has to carry out the above procedure again.
Thus the conventional roller-type blood pump requires a very complicated adjustment procedure, however, it is very dangerous and sometimes impossible to carry out such a complicated adjustment during an operation.